Heat and water management in fuel cells is a necessary aspect to attaining better cell efficiency and longevity. For proton exchange membrane (PEM) fuel cells with perfluorosulfonic acid (PFSA) type membranes, such as Nafion®, water management is a persistent challenge, where PEM fuel cells generally require high water activity for suitable ionic conductivity. Typically, humidification of the reactant gases ensures the proper humidification of the membrane. The oxygen reduction reaction at the cathode of a PEM fuel cell produces water in liquid form. Liquid water fills the pores of the catalyst layer and gas diffusion layer (GDL) and restricts diffusion of oxygen to the catalyst. The liquid water emerges from the GDL via capillary action, accumulates in gas channels, covers the GDL surface, increases the pressure differentials along flow field channels, and creates flow maldistribution and instability in systems with multiple parallel channels.
A common strategy to mitigate flooding is to employ serpentine channels (most commonly a small number of serpentine channels in parallel) for the cathode and to supply air flow rates large enough to force liquid water out of the system. These strategies act in concert as serpentine designs increase flow rate per channel, improving the advective removal of water droplets. Air is often supplied at a rate several times greater than that required by the reaction stoichiometry, increasing the oxygen partial pressure at the outlet. The larger applied pressure differentials required for these designs further reduce flooding since pressure drop reduces local relative humidity, favoring increased evaporation rates near the cathode outlet. The use of high flow rate and high pressure contributes to air delivery being one of the largest parasitic loads on fuel cells. Miniaturization of forced air fuel cells exacerbates this parasitic load issue as the efficiency of miniaturized pumps and blowers is typically much lower than that of macroscale pumps. The flooding challenge is exacerbated in planar air-breathing fuel cells where water removal from the cathode by forced convection is not applicable.
Parallel channels can reduce the pressure differential across the flow field by orders of magnitude compared to serpentine channels. A parallel channel design also simplifies flow field machining and can enable novel fabrication methods. However, truly parallel channel architectures are typically impractical as they are prone to unacceptable non-uniformity in air streams and catastrophic flooding. Typically, oxygen stoichiometries greater than 4 are necessary to prevent parallel channel flooding. Further, in-situ and ex-situ visualizations show that considerable flooding occurs in the GDL directly under the rib of the flow field irrespective of current density.
Several passive water strategies employ additional components to mitigate flooding. For example, a composite flow field plate was fabricated featuring a thin water absorbing layer and waste channels for removing liquid water from the oxidant channels. The design, however, did not offer improved power density due to a significant increase in the Ohmic losses introduced by the new components.
Active water management strategies in which applied pressure differentials actively transport liquid water out of or into a fuel cell are now emerging. A PEM fuel cell was presented that actively managed the water content of the electrolyte by supplying pressurized water to wicks that were integrated into the membrane. Further presented was an active water management method having a bipolar plate that is porous and has internal water channels for cooling and water removal. An applied pressure differential between the gas and water streams drives liquid water from the air channels and into internal channels dedicated to water transport.
Accordingly, there is a need to develop a passive heat and water management device and method for fuel cells that minimizes parasitic energy losses.